solr_facet_field_formatter/solr_facet_field_formatter.module

<?php

/**
 * @file
 * Contains facet_field_formatter.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function solr_facet_field_formatter_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the solr_facet_field_formatter module.
    case 'help.page.solr_facet_field_formatter':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Format string and text fields to link to Solr facet searches') . '</p>';
      return $output;

    default:
  }
}

/**
 * Implements hook_field_formatter_info_alter().
 *
 * Makes the Solr facet formatter available for every field type. The value
 * sent to the facet search is resolved per field type inside the formatter
 * (the referenced entity's label for reference fields such as entity_reference
 * and typed_relation, the field's main property value otherwise), so the facet
 * is filtered by the human readable value rather than an internal id.
 */
function solr_facet_field_formatter_field_formatter_info_alter(array &$info) {
  if (isset($info['solr_facet_text_field_formatter'])) {
    $field_types = array_keys(\Drupal::service('plugin.manager.field.field_type')->getDefinitions());
    $info['solr_facet_text_field_formatter']['field_types'] = $field_types;
  }
}